1. Field of the Invention
The present invention relates to integrated sensors, such as CMOS image sensors.
2. Description of the Related Art
CMOS image sensors suffer from the problem that, with an indirect band-gap, a relatively thick absorption region is often needed to achieve the desired quantum efficiency (defined as the ratio of the number of collected photo-electrons (or photo-holes) to the number of incident photons). Unfortunately, at deep sub-micron technologies, the absorption regions often cannot be made thick enough, especially for longer wavelength light.
FIG. 1 is a cross-sectional view illustrating part of an image sensor according to the prior art. As shown in FIG. 1, incident light 102 enters at one side of a light-sensing element (represented as absorption region 100), within which some of the incident light is absorbed at locations 104 and the remaining light 106 exits at the opposite side. Absorption region 100 could correspond to an epi layer on the front side of a silicon wafer, where a photodiode is formed within the epi layer. Since silicon is an indirect band-gap material, the absorption coefficient of visible light in the range of 400-700 nanometers in silicon is relatively low.
One way to increase the quantum efficiency of the light-sensing element is to make absorption region 100 thicker in order to absorb a greater percentage of the incident light. For example, at a wavelength of 700 nanometers, an epi layer thickness of about 15 microns would absorb about 99% of incident photons. For longer wavelengths important in optical communications (such as the near IR, where wavelengths can extend to 1 micron), the absorption region of the light-sensing element must be made even thicker to achieve the same quantum efficiency. Unfortunately, this technique cannot be applied in deep sub-micron technologies, where the absorption regions often cannot be made thick enough to achieve the desired quantum efficiency.
According to the present invention, one or more reflective elements are positioned below the absorption regions of light-sensing elements in image sensors and the like. In these devices, light that passes through the absorption region of a light-sensing element without being absorbed is reflected by a corresponding reflective element back towards the absorption region, such that the reflected light enters the absorption region a second time, giving those reflected photons a second chance at being absorbed and thereby increasing the effective quantum efficiency of the light-sensing element. In this way, the present invention provides a technique for improving quantum efficiency without having to increase the thickness of the absorption region. As such, more efficient image sensors can be implemented, including those implemented using deep sub-micron technologies.
According to one embodiment, the present invention is a light sensor comprising (a) a light-sensing element formed on a substrate, the light-sensing element designed to generate an electrical signal in response to incident light absorbed within the light-sensing element; and (b) a light-reflecting element positioned in relation to the light-sensing element to reflect light transmitted through the light-sensing element back towards the light-sensing element to increase an effective quantum efficiency of the light-sensing element.
According to another embodiment, the present invention is a method comprising the steps of (a) forming a light-sensing element on a substrate, the light-sensing element designed to generate an electrical signal when incident light is absorbed therein; and (b) positioning a light-reflecting element in relation to the light-sensing element to reflect light transmitted through the light-sensing element back towards the light-sensing element to increase an effective quantum efficiency of the light-sensing element.